1. Field of the Invention
The present invention relates to a method for fabricating semiconductor devices, and more particularly, to a method for removing a photoresist mask used for etching of a metal layer on a wafer as well as other etching by-products.
2. Description of the Related Art
As design rules required for fabrication of high-integrated semiconductor devices become tighter, high density plasma (HDP) equipment is employed as etching equipment when forming a metal pattern on a wafer by a photolithography and dry etch in order to achieve high throughput in case of mass-production.
Compared with medium density plasma (MDP) etching equipment, the HDP etching equipment operates at lower pressure with higher Radio Frequency (RF) power supply, thereby increasing the density of plasma and etching efficiency. When etching a metal layer using the HDP etching equipment, hard polymer which is a by-product of the etching is produced in great quantity. Such production of a great quantity of hard polymer particularly happens at edges of the wafer.
For example, in the HDP having the density of plasma of about 10.sup.13 /mm.sup.3, the density of ion or electron is larger, as compared with the MDP etching equipment having the density of plasma of about 10.sup.11 /mm.sup.3 or less. Accordingly, stronger energy is applied to a layer to be etched, as compared with the MDP etching equipment. The polymer as a by-product of etching which has been produced under the situation where such high energy is applied, adheres with a high bond energy to the side wall of a metal layer pattern and the side wall of a photoresist mask remaining on the metal layer pattern, thus forming hard polymer which cannot be decomposed at an energy less than the bond energy.
To remove the photoresist mask and other by-products of etching remaining on the metal layer pattern after the etching of the metal layer, generally, an ashing process is carried out under the atmosphere of oxygen or the atmosphere of a gas containing oxygen. Consequently, the interior of an ashing chamber is in the atmosphere of oxygen at a stabilization step for introducing the ashing process. Conventionally, the ashing process performed in the oxygen atmosphere comes immediately after the etching of the metal layer.
During the etching of the metal layer, particularly, the etching for forming an aluminum wire, chlorine gases such as BCl.sub.3, SiCl.sub.4, and Cl.sub.2 are used. Accordingly, by-products of the etching, such as C, Ti, Al, Si, Al.sub.x Cl.sub.y, Al.sub.x, C.sub.y, Ti.sub.x Cl.sub.y and Ti.sub.x C.sub.y, remain on the surface of the wafer immediately after the etching process. When the ashing process in the oxygen atmosphere is performed immediately after the etching process, as in the conventional technique, the by-products remaining on the wafer after the etching as mentioned above are exposed to the oxygen atmosphere so as to be oxidized in the interior of the ashing chamber during the stabilization step for introducing the ashing process. The oxidized by-products of the etching exist in the form of CO, CO.sub.x, Al.sub.x O.sub.y, Al.sub.x Cl.sub.y O.sub.z, Al.sub.x C.sub.y O.sub.z, Ti.sub.x O.sub.y and Ti.sub.x Cl.sub.y O.sub.z and form the hard polymer which cannot be removed during the ashing and strip processes.
FIGS. 1A to 1C are sectional views which illustrate what occurs when the metal layer is dry-etched using a photoresist mask and then the photoresist mask is removed by the ashing and strip processes according to the prior art. Referring to FIGS. 1A and 1B, when a metal layer 12, e.g., an aluminum layer, on a semiconductor substrate 10 is dry-etched using a photoresist mask 14 in the HDP etching equipment, hard polymer 20 is formed as a by-product of the etching and adheres to the side wall of a metal layer pattern 12a and the side wall of a photoresist mask 14a remaining on the metal layer pattern 12a. Additional etching by-products, which act as sources for forming the polymer, remain around the hard polymer 20. These additional etching by-products, such as substances 30 (hereinafter, called "a polymer source 30"), typically including, e.g., C, Ti, Al, Si, Al.sub.x Cl.sub.y, Al.sub.x C.sub.y, Ti.sub.x Cl.sub.y and Ti.sub.x C.sub.y, are physically adsorbed to the hard polymer 20.
In case that the wafer with the hard polymer 20 and the polymer source 30 on the semiconductor substrate 10 is moved into the ashing chamber and then goes through the ashing and strip processes according to the prior art as mentioned above, the hard polymer 20 and the polymer source 30 remaining around the hard polymer 20 in the physically adsorbed state are exposed to the oxygen atmosphere and oxidized into CO, CO.sub.x, Al.sub.x O.sub.y, Al.sub.x Cl.sub.y O.sub.z, Al.sub.x C.sub.y O.sub.z, Ti.sub.x O.sub.y, Ti.sub.x Cl.sub.y O.sub.z and the like in the ashing chamber at the stabilization step performed prior to the ashing of the photoresist mask 14a. Among the oxidized substances noted above, many have very high evaporation points, particularly, Al.sub.x Cl.sub.y O.sub.z, Al.sub.x C.sub.y O.sub.z and Ti.sub.x O.sub.y, which have evaporation points of 192.degree. C., 2100.degree. C. and 579.degree. C., respectively. Accordingly, the hard polymer 20 adhering to the side wall of the metal layer pattern 12a and the side wall of the photoresist mask 14a becomes difficult to remove by the conventional ashing and strip methods.
As a result, after the ashing and strip processes, the hard polymer 20 on the side wall of the photoresist mask 14a is not removed and remains on the metal layer pattern 12a as shown in FIG. 1C even though the hard polymer 20 on the side wall of the metal layer pattern 12a is removed together with the photoresist mask 14a. The reason that only the hard polymer 20 on the side wall of the metal layer pattern 12a is removed while the hard polymer 20 on the side wall of the photoresist mask 14a remains is as follows. Polymers produced when a metal layer, e.g., an aluminum layer, is etched are in the form of Al.sub.x Cl.sub.y, Al.sub.x C.sub.y, Ti.sub.x C.sub.y and the like. Such polymers may adhere to the aluminum, but have greater affinity for a photoresist than the aluminum. Accordingly, the hard polymer such as Al.sub.x C.sub.y or Ti.sub.x C.sub.y adheres to the surface of the photoresist mask 14a much more. Therefore, the hard polymer 20 which has continuously adhered to the side wall of the photoresist mask 14a from the early stage of the etching is not easily removed during the ashing process.
As described above, in case of removing the photoresist mask used during the etching of the metal layer according to the prior art, the hard polymer remains on the metal layer pattern even after the ashing process. Consequently, surface resistance on the metal layer increases and an interlayer insulating film formed on the metal layer is severely contaminated, thereby deteriorating the quality of devices. This make mass production difficult.